The present invention relates to disk drive systems. In particular, the invention relates to a shielded flexible circuit for an actuator arm that supports a magnetic data read/write head and a servo head of a hard disk storage device.
Designers of prior art hard disk storage devices have encountered difficulties in mounting a servo head and a data head on the same actuator arm. The signals going to the data head are believed to interfere with the servo signals from the servo head, which can cause inaccuracy in the positioning of the actuator arms and attached heads. Because of this interference, disk storage device designers typically have placed the servo surface on either of the outermost surfaces in the disk stack. With this servo surface placement, the servo head is mounted by itself on an outermost actuator arm, and no signal interference occurs.
Actuator arms and attached heads can sometimes become misaligned with the tracks. In disk storage devices having a servo surface positioned at either end, the actuator arms mounted farthest away from the servo surface are subject to greater alignment error than those mounted closer to the servo surface. A more complete discussion of this problem is found in Anderson et al. U.S. Pat. No. 4,402,025 at column 1.
Placement of the servo surface in the middle of the disk stack is known to reduce certain types of alignment error, but it magnifies other sources of error such as crosstalk and noise from the data heads, which disrupt the servo signal. The physical closeness of the data preamplifier conductors and the servo preamplifier conductors results in an unacceptable amount of crosstalk between the data signals and the servo signals. The data write signals are typically high current signals. In contrast, the servo signals are typically of a much lower current. The high current data write signals can interfere with the low current servo signals.
When the servo surface is placed in the middle of the disk stack, the data surface adjacent to the servo surface must either be accessed by mounting a data head on the same actuator arm as the servo head, or that data surface must be left unused. In order to maximize the capacity of a disk storage device, it is desirable to mount a servo head and a data head on the same actuator arm.
Designers of hard disk storage devices have attempted to achieve adequate disk storage device performance with servo heads and data read/write heads positioned on the same actuator arm by providing shielding in the circuitry of the actuator arm. For example, Anderson et al U.S. Pat. No. 4,402,025 discloses a multi-part magnetic shield in a magnetic disk drive head/actuator arm assembly for preventing interference between data signals and the position (servo) signals. The shield is positioned on the actuator arm between a magnetic read/write head and a servo head.
Anderson et al also discloses a multilayer flexible circuit for use in an actuator arm supporting both a servo head and a data read/write head. The servo and data circuitry are shielded by spacing the servo preamplifier apart from the read/write circuitry, and by placing shielding traces on either side of the servo traces as well as on either side of the read/write traces. A ground plane is positioned between the servo and data conductive traces.
Another magnetic shield known in the art is a foil conductor formed from aluminum which is attached to and completely covers the lower surface of a flexible circuit. On the upper surface is a servo preamplifier and a data preamplifier. The shield is attached to the flexible circuit with an adhesive.
There are several disadvantages in using magnetic shields. First, the magnetic shield requires another piece part and an additional assembly step or steps to install the shield.
Second, a magnetic shield is also undesirable because the shield and adhesive both add weight to the actuator arm, which increases its mass moment of inertia. Any increase in the mass moment of inertia of an actuator arm adversely affects disk drive performance. As the inertia increases, the structure cannot be accelerated as easily and therefore the actuator arm's response time increases. Power consumption of the disk drive also increases with an increase in the actuator arm inertia. The structural requirements of the actuator arm also must be increased to withstand the increased torsional forces.
Another technique for reducing noise and crosstalk from mounting a data head and a servo head on the same actuator arm is to use a conductive shield layer of a defined shape within a multilayer flexible circuit to shield certain portions of the circuitry. The shield layer interacts with various features of the circuit design to create a low noise environment.
Some known design features which minimize crosstalk and noise include separating the signal paths, routing the ground paths in a manner which reduces noise, and noise cancellation by shielding portions of custom hybrid circuits.
It is known to use a shield layer in a multilayer flexible circuit for use as part of a radio frequency (RF) circuit design for reducing noise and crosstalk on a servo/data flexible circuit. This prior art flexible circuit has a shield layer of a substantially L-shape, for shielding custom hybrid components having a particular shape, such that the data input and output pins to the data preamplifier and the servo input pins to the servo preamplifier are shielded. This prior art flexible circuit also shields a portion of the data lines which are electrically connected to the data input pins, data output pins, and servo input pins. The shield layer covers particular portions of the flexible circuit and relies on complex radio frequency interaction to create feedback having the desired noise cancellation effects.
There are several disadvantages in using the above-mentioned prior art flexible circuit. The circuit design requires custom hybrid parts, which typically are significantly more expensive than standard parts, and are less reliable. In order to adequately reduce noise, the prior art flexible circuit requires a metal outer layer. Metal layers are expensive to fabricate, are more difficult to clean, and add mass to the actuator arm.